Abstract/Summary

In temperate Atlantic waters (18.8 to 20.1°C), biological nitrogen fixation has been
demonstrated by 2 independent measurements: 15N-N2 incorporation and nifH identification in the
DNA and expressed messenger RNA (mRNA). At 2 stations in the western English Channel, bulk
waters were incubated with 15N-N2. At the high levels of particulate nitrogen (≤11.5 μmol N l–1),
absolute fixation rates of 18.9 ± 0.01 and 20.0 nmol N l–1d–1 were determined. While a caveat must
accompany the magnitude of the rates presented due to the limited number of data, the presence and
activity of diazotrophic organisms in these waters is of ecological significance and may affect current
attitudes to nitrogen and carbon budgets. In particular, our estimate of the rate of N fixation
(0.35 mmol N m–2 d–1) is comparable to that of denitrification rates in UK shelf seas. Molecular analysis
identified a diversity of expressed nifH genes, and 21 different prokaryotic nifH transcripts were
identified.